There has been conventionally known a variable geometry turbocharger in which an opening area of a nozzle for ejecting exhaust gas into an exhaust turbine can be adjusted. In such a variable geometry turbocharger, at a low speed revolution zone of an engine having a small amount of exhaust gas, a gap between exhaust inlet walls forming a nozzle is reduced to reduce an opening area thereof, thereby increasing a flow speed of exhaust gas flowing into the exhaust turbine. Thus, the rotary energy of a turbine can be increased to enhance supercharging performance of a charging compressor.
It has been proposed that a slide mechanism in which one of the exhaust inlet walls is slid toward the other exhaust inlet is employed for adjusting the gap between the exhaust inlet walls, and the slide mechanism is driven by a hydraulic servo drive device (for example, Patent Document 1). In such a hydraulic servo drive device, a hydraulic servo piston is reciprocated by switching oil supply for hydraulic chambers provided on both sides of the hydraulic servo piston to transfer this reciprocating motion to the slide mechanism, thereby controlling an opening degree.
However, in Patent Document 1, when a stroke sensor and the like are attached to the hydraulic servo drive device provided in the variable geometry turbocharger, the turbine is heated to a high temperature. Accordingly, the stroke sensor and the like which are vulnerable to heat may be damaged from the heat of the turbine. For this reason, it has been known that a cooling path or space is provided only for cooling the stroke sensor in, for instance, a hydraulic servo drive device provided in an EGR (Exhaust Gas Recirculation) valve device (Patent Documents 2 to 4).    Patent Document 1: JP-T-2003-527522    Patent Document 2: JP-A-7-190227    Patent Document 3: JP-A-2000-282964    Patent Document 4: JP-A-2007-107389